I'd say Mono/Xamarin and .NET/Microsoft would qualify as competing production quality implementations of C#. Xamarin may have positioned their implementation of

I'd say Mono/Xamarin and .NET/Microsoft would qualify as competing production quality implementations of C#. Xamarin may have positioned their implementation of C# to target the mobile space instead of the enterprise space because they see that as a losing battle against Microsoft (and the niche they do address is very lucrative right now) but the could still qualify as competitors.